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Power-Efficient ELF Wireless Communications Using Electro-Mechanical Transmitters

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Power-Efficient ELF Wireless Communications Using Electro-Mechanical Transmitters Received November 20, 2019, accepted December 18, 2019, date of publication December 23, 2019, date of current version January 6, 2020. Digital Object Identifier 10.1109/ACCESS.2019.2961708 Power-Efficient ELF Wireless Communications Using Electro-Mechanical Transmitters JARRED S. GLICKSTEIN 1, (Student Member, IEEE), JIFU LIANG 1, (Member, IEEE), SEUNGDEOG CHOI 2, (Senior Member, IEEE), ARJUNA MADANAYAKE 3, (Senior Member, IEEE), AND SOUMYAJIT MANDAL 1, (Senior Member, IEEE) 1Department of Electrical Engineering and Computer Science, Case Western Reserve University, Cleveland, OH 44106, USA 2Department of Electrical and Computer Engineering, Mississippi State University, Starkville, MS 39762, USA 3Department of Electrical and Computer Engineering, Florida International University, Miami, FL 33174, USA Corresponding author: Jarred S. Glickstein ([email protected]) ABSTRACT Ultra-miniaturized and highly power-efficient ELF (0.3-3 kHz) transmitters are desirable for underground and undersea wireless communications. This paper proposes the use of rotating permanently polarized dipoles (magnets or electrets) for such applications. A power-efficient data modulation scheme based on continuous-frequency FSK (CF-FSK) is also proposed for such electro-mechanical transmitters. Theoretical analysis and simulations show that 7-ary CF-FSK is optimal in the sense that it minimizes average mechanical torque on the mechatronic antenna for a given bit rate. Preliminary experimental results from a prototype ELF magnetic-field based transmitter based on a high-strength rare-earth (NdFeB) magnet and small brushless DC (BLDC) motor are presented. The results highlight local environment challenges such as power line interference that affect the choice of data encoding and modulation schemes. Reliable non-line- of-sight (NLOS) communication through concrete barriers at 100 Hz and ∼0.25 bit/sec is demonstrated at distances up to ∼5 m. The proposed mechatronic antenna and modulation scheme can be scaled up in both frequency and transmit power for more challenging applications in surface-to-undersea, surface-to-cave, and other conductive environments that require ELF channels. INDEX TERMS Antennas and propagation, communication systems, electromechanical devices, emergency services, magnetic devices, transmitters. I. INTRODUCTION allows undersea communications to depths of ∼30 m with Wireless communications between the earth's surface and reasonable transmit power levels [2]. underground or deep-water facilities (mines, shelters, stor- The availability of portable low-power ELF transceivers age areas, tunnels, submarines, undersea cables, etc.) is would immediately enable communications within such very difficult due to the short skin depth δ of electromag- RF-denied environments by enabling bidirectional low-data- netic (EM) waves within conductive media such as earth rate wireless links with the surface. Promising applications or seawater,p which leads to high attenuation. Fortunately, of such transceivers include oil and gas exploration, search δ / 1= ! where ! is the frequency. Thus, extremely low and rescue missions (cave rescues including when there frequency (ELF) EM waves can penetrate long distances is significant bodies of water in the signal path under- in RF-denied environments [1]. Note that for convenience, ground), biosensing (e.g., of marine and underground life- here we use ``ELF'' as shorthand for the combination of forms), underwater and underground Internet of Things (IoT) the ELF, SLF, and ULF ranges (3-30 Hz, 30-300 Hz, and networks (e.g., for ocean pollution monitoring and soil 0.3-3 kHz, respectively), for which EM wavelengths range sensing), inter-submarine and submarine to surface wire- from 105 − 102 km in air. For example, the skin depth in sea less links, underwater autonomous robotics, advanced marine water (conductivity σ ≈ 5 S/m) is δ D 7:1 m at 1 kHz, which biology experiments, geophysics (e.g., subsurface imaging for locating geological conductors, such as ore deposits) The associate editor coordinating the review of this manuscript and [3] and atmospheric science (e.g., studies of the iono- approving it for publication was Cunhua Pan . sphere) [4], [5]. The resulting near-field links have EE and This work is licensed under a Creative Commons Attribution 4.0 License. For more information, see http://creativecommons.org/licenses/by/4.0/ VOLUME 8, 2020 2455 J. S. Glickstein et al.: Power-Efficient ELF Wireless Communications Using Electro-Mechanical Transmitters BE fields that propagate independently; here we analyze and those from oscillating electrical currents inside conductors) experimentally demonstrate magneto-inductive (MI) digital are generated by periodic mechanical motion of a magnet or wireless links [6] based only on BE fields, because magnetic electret. This ``all-mechanical transmitter'' approach is more fields are much less shielded than EE fields by dielectric power-efficient at low frequencies than conventional elec- objects in the medium [7]. trical transmitters, as analyzed in our earlier work [1], [2]. However, near-field ELF links present several problems, It also differs from early electromechanical transmit- including i) limited bandwidth due to the use of low frequen- ters, which simply coupled high-frequency AC generators cies, which in turn limits channel capacity; and ii) the need (e.g., the Alexanderson alternator [20]) to dipole or loop to use electrically small antennas due to device size con- antennas. straints. Fundamentally, the Chu-Harrington limit [8]–[10] Electro-mechanical ELF transmitters have received signif- lower-bounds the quality factor (Q) of small antennas to icant academic attention since the concept was first described ∼ (c=!a)3, where a is the radius of its enclosing sphere, in 2017 as part of the DARPA AMEBA program [21]. Proto- As a result, the bandwidth and radiation efficiency of small types of transmitters based on rotating magnetic dipoles were dipoles both degrade rapidly with frequency (as !4a3 and demonstrated in [22], [23]. In both cases, the authors mea- !3=2a, respectively), while small loops are even worse [2]. sured near-field antenna patterns and dependence of signal Thus, extremely large transmit antennas are necessary for strength versus distance, but did not discuss data transmis- obtaining reasonable bandwidth, channel capacity, and radi- sion. Other work has demonstrated all-mechanical very low ation efficiency at ELF [11]–[13]. ELF receivers can avoid frequency (VLF) transmitters by using piezoelectric elements similar issues by relying on noise matching (rather than to produce oscillating electric fields [24], [25]. The generated impedance matching) to get high sensitivity. In particu- VLF electric fields have either directly been used to transmit lar, broadband noise matching is possible even for high- information [24], or coupled to a magnetostrictive material Q source impedances [14], which allows ELF receivers to generate a magnetic field [25]. These works again pay a with modestly-sized antennas to obtain excellent sensitivity great deal of attention to design of the proposed antenna, but (< 0:1 fT/Hz1=2) over a broad frequency range [15]–[17]. do not consider data transmission. In this paper, we empha- Thus, miniaturized and highly-sensitive ELF receivers are size suitable data modulation and encoding schemes, available, while far-field transmitters remain physically large and also discuss how they influence the design require- and extremely power-hungry. Fortunately, the issue of poor ments for ELF transmitters based on mechanically-rotating radiation efficiency can be avoided in the near-field, where dipoles. the radiated fields are small compared to the stored fields. The paper is organized as follows: Section II presents a In fact, various switching-based direct antenna modula- theoretical analysis of the proposed ELF transmitters. Sys- tion (DAM) schemes have been proposed for decoupling tem design and data modulation are discussed in a theoret- the efficiency, bandwidth, and Q of near-field antennas [18], ical context in Section III, and in an experimental context [19]. Hence this paper focuses on miniaturized near-field ELF in Section IV. A prototype mechanical antenna is used in transmitters, which are the key for realizing short-range (up Section V to examine practical efficacy of the proposed data to ∼1 km) bidirectional wireless links in conductive media. transmission system. Finally, Section VII concludes the paper Since near-field transmitters are not limited by antenna and discusses directions for future work. bandwidth or Q, the key issue for practical implementations is simply field generation efficiency, which is defined as II. THEORETICAL ANALYSIS the magnitude of BE per unit power consumption at a given A. FIELD GENERATION distance r, and has units of T/W. Modern ferromagnetic The proposed ELF transmitters generate oscillating BE fields and ferroelectric materials (permanent magnets and electrets, by physically moving one or more static field sources. respectively) have extremely high energy density and dis- While in principle any type of periodic motion can be used, play strong permanent dipole moments, which makes them in practice mechanical challenges restrict the main choices to promising for achieving high field generation efficiency [1]. oscillations or rotations about a single axis [22]–[25]. Here For example, let us compare a cylindrical NdFeB grade we focus on rotating magnetic or electric dipoles, which N52 magnet (remanent field Br D 1:5 T, length
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